May 10, 2011 -- Women who drink more than five cups of coffee a day may be reducing their risk of one type of breast cancer, new research suggests.
Previous research has produced conflicting results about coffee and breast cancer risk, says researcher Jingmei Li, PhD, of the Karolinska Institutet in Stockholm.
In her new research, she found coffee drinking reduces overall breast cancer risk modestly -- by 20% -- when she considered age. "The 20% decrease in risk associated with drinking five or more cups of coffee a day was statistically significant only when adjusted for age," she tells WebMD.
When she took into account other factors, such as education level, drinking of alcohol, and hormone therapy use, she found a 57% reduction in risk for cancers known as estrogen-receptor negative cancers. This type of breast cancer is less likely to respond to hormone therapy than estrogen-receptor positive breast cancer.
However, a U.S. expert warns that the new finding about reduction in risk for ER-negative breast cancer could be due to chance. The only solid message from this study and previous ones, says Shumin Zhang, MD, ScD, a researcher at Harvard Medical School, is this: "Drinking coffee doesn’t seem to increase the overall risk of breast cancer."
The researchers from Karolinska Institutet evaluated coffee drinking and breast cancer risk in 2,818 patients with breast cancer and 3,111 study participants who did not have breast cancer.
The breast cancer patients were classified by estrogen-receptor tumor subtypes.
Breast cancer cells are termed ER-negative if they don't have receptors for estrogen. They are ER-positive if they do. Receptors are proteins on the outside surfaces of cells that can attach to hormones found in the blood. When estrogen attaches, it can fuel the growth of breast cancer cells.
Participants were ages 50 to 74, all Swedish born and residents there between October 1993 and March 31, 1995.
The researchers collected information on coffee drinking habits. They also asked about education, family history of breast cancer, menstrual history, reproductive history, and habits such as smoking, drinking alcohol, and exercising.
Coffee drinkers were grouped into four categories:
One cup or less a day
More than one cup and up to three cups a day
More than three cups and up to five cups a day
Five or more cups a day
Those who had one cup or less a day served as the reference group.
''The results are biologically plausible," Li says. Coffee has compounds that may affect breast cancer of different ER subtypes in different ways, she tells WebMD.
For instance, she says, coffee has been shown to boost blood levels of the phytoestrogen enterolactone. It is linked in other studies with a decrease in ER-negative breast cancer risk, she says.
In her own research, Zhang has found that several cups of coffee daily, overall, do not seem to pose a risk for breast cancer.
However, in research published in 2008, she found coffee intake was linked with an increased risk of ER-negative cancers -- exactly the opposite of the new study.
Li's finding that heavy coffee drinking may reduce ER-negative breast cancer risk, she says, is interesting. However, she isn't convinced. "It is unclear if this observation was a chance finding."